The present invention relates to a switched sensing technique and more specifically to an apparatus and method for improving load regulation in switching power supplies which typically employ ballistic sense windings.
Switching power supplies, incorporating transformers about which a primary winding and a secondary winding are wrapped, can be particularly useful for applying power to different electrical loads. In certain applications, it is necessary to provide regulated voltage levels to the loads. To achieve such regulation, the power supply may include a regulator circuit employing feedback to control current and/or voltage.
The feedback can be obtained by directly sensing the output. For example, in U.S. Pat. No. 4,566,060 to Hoeksma, output is supplied to the load and a signal from the load is consequently transmitted across an isolating feedback member. The feedback member preferably includes an optocoupler, which may include a light-emitting diode and a photo-transistor. While this technique of load regulation is effective, it is disadvantageous from a cost viewpoint since an optocoupler must be employed to cross the barrier between the primary winding and the secondary winding.
Ballistic sense windings have been used to provide an inexpensive technique for regulating the outputs of switching power supplies. In the typical circuit employing the ballistic sense winding, good coupling can generally be achieved between the primary winding and the secondary winding, as well as between the primary winding and the ballistic winding. Due to the good coupling between the primary and ballistic windings, voltage overshoot occurring on the primary winding, in response to leakage between the primary and secondary windings, may occur on the ballistic winding. This can lead to severe load regulation problems since overshoot in the ballistic winding leads to improper control of the primary winding and hence poor regulation of output voltage.
There are at least two known techniques having the object of alleviating load regulation problems arising as a result of overshoot in the ballistic winding. First, a resistor or inductor is used in series with the ballistic winding to filter out the overshoot. The filtering resistor or inductor is generally sized to compensate for peak load cases in which overshoot is most severe. Consequently, for minimum loads, the filtering resistor or inductor not only filters out the overshoot, but also has an averaging effect on the entire ballistic waveform. This averaging effect can lead to inadequate load regulation, particularly when the power supply must function from no load to full load.
Second, the regulating circuit can be designed to compensate for overshoot. For example, U.S. Pat. No. 4,658,342 to Wharton, is directed toward a circuit that compensates the duty cycle of a transistor, which is coupled to the primary winding, in response to load changes. In particular, duration of the transistor duty cycle depends on a composite feedback signal which is generated by the ballistic winding operating in conjunction with two filter circuits. As circuit loading increases, the composite feedback signal is used to increase the conduction time of the transistor to hold the outputs within regulation requirements.
It is believed that neither of the above prior art techniques employing ballistic windings represent effective or economical solutions to problems arising as a result of overshoot caused by leakage inductance. While the technique in which a series resistor or inductor is used represents an economical approach, it has the above-discussed undesirable effect on the ballistic waveform. The Wharton arrangement, which is not very economical, may not even be consistently effective in regulating load since the Wharton arrangement does not eliminate overshoot. In particular, while the Wharton approach of prolonging the duty cycle of the transistor to compensate overshoot may be sound in theory, the amount of overshoot encountered varies from load to load and it is not clear how the interval over which duty cycle must be prolonged can be readily set for each of the many increases or decreases in load.